i was the measured current due to the charge carried by the α rays.
From the magnetic deflection, it is known that
(m/e)v = 3·98 × 10^5.
From these two equations we obtain
v = 2·6 × 10^9 cms. per second.
e/m = 6·5 × 10^3 electro-magnetic units.
These values are in surprisingly good agreement with the previous values of the writer and Des Coudres (section 91). On account of the uncertainty attaching to the value of n, not much weight can be attached to the determination by this method of the constants of the α particles.
Decrease of velocity of the α particles in passing through matter. Some experiments were made to determine the velocity of
the α particles from radium C after passing through known thicknesses
of aluminium. The previous apparatus was employed, and the distance
between the photographic bands was observed for successive layers of
aluminium foil, each ·00031 cms. thick, placed over the active wire.
The photographic plate was placed 2 cms. above the slit, and the
magnetic field extended 1 cm. below the slit. The amount of deviation
of the rays is inversely proportional to their velocity after
traversing the aluminium screens. The impressions on the plate were
clear and distinct, and about the same in all cases, showing that the
rays were still homogeneous after passing through the aluminium.
A clear photographic impression was obtained for 12 layers of foil, but it was not found possible to obtain any effect through 13 layers. This result shows that the photographic action of the rays, like the ionizing action, ceases very abruptly.
The results obtained are shown in the following table. Assuming that the value of e/m is constant, the third column gives the velocity of the α particles after traversing the aluminium. This is expressed in terms of V_{0}, the velocity of the α particle when the screens are removed.
Number of layers Distance between bands Velocity of
of aluminum foil on the plate α particles
0 1·46 mms. 1·00 V_{0}
5 1·71 " ·85 "
8 1·91 " ·76 "
10 2·01 " ·73 "
12 2·29 " ·64 "
13 No photographic effect
